This grant proposes to collect mechanistic information on and understand the regulation of D-arabinose 5- phosphate (ASP) isomerase (API), the enzyme responsible for the synthesis of ASP from D-ribulose 5- phosphate (Ru5P), in Gram-(-) microorganisms. This information should provide validation that API, a key enzyme in lipopolysaccharide (LPS-aka endotoxin) biosynthesis, is a target for screening efforts by investigators in the field to identify selective inhibitors of API - namely a new generation ofmechanistically diverse antibiotics for which no resistance is known. The goals of this project are to establish 1. the intracellular function of the ASP isomerases expressed by the genes yrbH, kpsF, c3406 and gutQ as well as to understand the regulatory role of the cystathionine beta -synthase domain (CBS) common to three of the gene products;2. the mechanism for the formation of ASP from RuSP;and 3. the substrate specificity of the APIs and the role of active site amino acids. The specific aims focus on diverse techniques to detect the presence of an intermediate enediol and to determine the stereochemistry of the inter-conversion of the substrate and product. Genomic knockouts will be constructed to understand the cellular function of each API as well as the potential lethality of a genomic knockout of API (target validation). Truncated derivatives of API lacking the CBS domain will be utilized to ascertain CBS domain function and a library of sugar nucleotides will be screened to find potential regulators of the CBS domain. Site-directed mutagenesis studies, based on crystallographic data and active site modification using mechanism-based irreversible inhibitors, will be exploited to gain insight into the contribution of enzyme functionalities to substrate binding, monomer interface interactions, and to the mechanism of API. The small molecules to be synthesized in this grant will serve as mechanistic probes. The ultimate goal of these studies is to better understand the role of LPS, a critical macromolecule essential to both the survival and virulence of Gram-(-) microorganisms